window.addEventListener('load', function () {
    // 0. nav下拉菜单
    var arrow_icons = document.querySelectorAll('.arrow-icon');
    for (var i = 0; i < arrow_icons.length; i++) {
        arrow_icons[i].onmouseover = function () {
            this.children[1].style.display = 'block';
        }
        arrow_icons[i].onmouseout = function () {
            this.children[1].style.display = 'none';
        }
    }
})